"One of the biggest trends since middle of last year is marketers wanting to get more involved in their activity programmatically," Brunick said. "They want to do strategic and long-term deals with media owners they've wanted to work with."

Unbound was founded by two former Mediabrands executives, Brendan Moorcroft and Quentin George. The three men developed Mediabrands' Cadreon trading desk together, so "the gang is all back together," Moorcroft said in a note to AdExchanger.

Mediabrands confirmed Brunick's departure and said a search is underway for his replacement. He leaves after a 14-year stretch with the company, an almost unheard of tenure in agency circles. His first job out of college was with an IPG agency, Universal McCann.

The timing of Brunick's exit is awkward for Mediabrands, coming in the wake of two other high-profile departures and losses of key clients Microsoft and Johnson & Johnson.

Mediabrands North America CEO Jacki Kelley recently left to join Bloomberg Media, after focusing for several months on the Microsoft review. And Magna Global's president for North America, Kristi Argyilan, quit for an opportunity leading Target's paid, owned and earned media activities.

Argyilan – along with Brunick – had been tasked with meeting Mediabrands' ambitious goal to automate 50% of all media buying by the end of 2015. Now others will have to run with that baton.

Unbound bills itself as a consultancy and systems integrator focused exclusively on the marketing technology arena.

"Our proposition is that both the brand marketer and the media owner need to treat programmatic like any other enterprise technology stack," George told AdExchanger last year. "They need to do a vendor ID process, they need to create functional abstraction and they need to connect it to the data and systems that are inside the enterprise."